try it with a .cjs resolver script

.parcelrc

@@ -1,5 +1,5 @@
 {
   "extends": ["@parcel/config-default"],
-  "resolvers": ["./scripts/parcel-resolver-locales.mjs", "..."],
+  "resolvers": ["./scripts/parcel-resolver-locales.cjs", "..."],
   "reporters": ["...", "parcel-reporter-static-files-copy"]
 }

.prettierignore

@@ -1,3 +1,4 @@
 # Build products
 build
 dist
+generated

scripts/parcel-resolver-locales.mjs → scripts/parcel-resolver-locales.cjs

@@ -1,8 +1,8 @@
-import { Resolver } from '@parcel/plugin';
-import { exec } from 'child_process';
-import * as path from 'path';
-import * as fs from 'fs';
-import { promisify } from 'util';
+const { Resolver } = require('@parcel/plugin');
+const { exec } = require('child_process');
+const path = require('path');
+const fs = require('fs');
+const { promisify } = require('util');
 
 async function allXlfFiles(projectRoot) {
   const entries = await promisify(fs.readdir)(
@@ -18,7 +18,7 @@ async function allXlfFiles(projectRoot) {
  * NB: this is not a Typescript file! (Parcel doesn't support plugins written
  * in TS.) No type checking!
  */
-export default new Resolver({
+module.exports = new Resolver({
   async resolve({ specifier, options: { projectRoot } }) {
     if (specifier.startsWith('locales:')) {
       await promisify(exec)('npx lit-localize build');